Polishing method

ABSTRACT

A semiconductor wafer (W) and a polishing table ( 100 ) are rotated. The polishing table ( 100 ) has a polishing surface thereon. The semiconductor wafer (W) is pressed against the polishing surface on the polishing table ( 100 ) rotated at a first rotational speed to polish the semiconductor wafer (W). The semiconductor wafer (W) is separated from the polishing surface after the semiconductor wafer (W) is pressed against the polishing surface. Before the semiconductor wafer (W) is separated from the polishing surface, a rotational speed of the polishing table ( 100 ) is reduced to a second rotational speed lower than the first rotational speed to provide a difference between a rotational speed of the semiconductor wafer (W) and the rotational speed of the polishing table ( 100 ).

TECHNICAL FIELD

The present invention relates to a polishing method, and moreparticularly to a method of polishing a workpiece such as asemiconductor wafer to a flat mirror finish.

BACKGROUND ART

As semiconductor devices have become more highly integrated in recentyears, circuit interconnections have become finer and distances betweenthose circuit interconnections have become smaller. In the case ofphotolithography, which can form interconnections that are at most 0.5μm wide, it is required that surfaces on which pattern images are to befocused by a stepper should be as flat as possible because the depth offocus of an optical system is relatively small. However, conventionalapparatuses for planarizing semiconductor wafers, such asself-planarizing CVD apparatuses and etching apparatuses, cannot producesemiconductor wafers having completely planarized surfaces. Recently, ithas been attempted to planarize semiconductor wafers with a polishingapparatus, which is expected to achieve complete planarization ofsemiconductor wafers more easily as compared to the above conventionalapparatuses.

FIG. 1 shows a basic arrangement of this type of polishing apparatus. Asshown in FIG. 1, the polishing apparatus has a polishing table 151having a polishing pad 161 thereon and a top ring 152 for holding asemiconductor wafer W as a workpiece to be polished in a manner suchthat a surface to be polished faces the polishing pad 161. The polishingpad 161 has an upper surface which serves as a polishing surface forpolishing a workpiece. The top ring 152 is connected to a lower end of atop ring shaft 153 via a ball joint 154 in a manner such that the topring 152 is tiltable with respect to the top ring shaft 153. The topring 152 presses the semiconductor wafer W against the polishing table151 under a desired pressure while the polishing table 151 and the topring 152 are independently rotated. A polishing liquid Q such asabrasive liquid or pure water is supplied onto the polishing pad 161from a polishing liquid supply nozzle 155. Thus, a surface of thesemiconductor wafer W is polished by the polishing liquid Q. At thattime, the surface of the semiconductor wafer W is brought into slidingcontact with a surface of the polishing pad 161 so as to follow thesurface of the polishing pad 161 via the ball joint 154.

There has heretofore employed a polishing pad made of non-woven fabricas a polishing pad attached to a polishing table. Higher levels ofintegration achieved in recent years for ICs and LSI circuits demandsmaller steps or surface irregularities on a surface of a semiconductorwafer. In order to meet such a demand, there has been used a polishingpad made of a hard material such as polyurethane foam.

After the semiconductor wafer W is thus polished by the polishingapparatus, it is necessary to remove (or separate) the semiconductorwafer W from the polishing surface (polishing pad 161) on the polishingtable 151. However, a large surface tension acts between the polishingpad 161 and the semiconductor wafer W due to the polishing liquid Qinterposed therebetween. Accordingly, if the top ring 152 holding thesemiconductor wafer W is lifted at a polishing position in order toremove the semiconductor wafer W from the polishing pad 161, there aresome cases in which only the top ring 152 is lifted and thesemiconductor wafer W adheres to the polishing pad 161 so as to be lefton the polishing pad 161.

Such a problem can be solved by an overhanging action of the top ring.In the overhanging action, after a polishing process is completed, thetop ring 152 is not lifted at the polishing position, but is moved to anouter circumferential edge of the polishing pad 161 to partly expose apolished surface of a semiconductor wafer W beyond the outercircumferential edge of the polishing pad 161, and is then lifted toremove the semiconductor wafer W from the polishing pad 161. Thisoverhanging action allows surface tension between the polishing pad 161and the semiconductor wafer W to be reduced, and the semiconductor waferW can reliably be removed or separated from the polishing pad 161.

As described above, the overhanging action can reduce the surfacetension between the polishing pad 161 and the semiconductor wafer W.However, the top ring 152 may tilt when the polished semiconductor waferW projects from the outer circumferential edge of the polishing pad 161.In such a case, the semiconductor wafer W is intensively pressed at theouter circumferential edge of the polishing pad 161, so that thesemiconductor wafer W is cracked or scratched.

Polishing capability of a polishing pad is gradually deteriorated due todeposits of abrasive particles and polishing wastes of semiconductormaterial, and due to changes in characteristics of the polishing pad.Therefore, if the same polishing pad is used to repeatedly polishsemiconductor wafers, a polishing rate of the polishing apparatus islowered, and polished semiconductor wafers tend to suffer polishingirregularities. Therefore, it has been customary to condition thepolishing pad according to a dressing process of recovering a surface ofthe polishing pad with a diamond dresser or the like before, after, orduring polishing.

When a dresser dresses a polishing surface of a polishing pad, itscrapes a thin layer off the polishing pad. Therefore, after thepolishing surface of the polishing pad has been dressed many times, itbecomes irregular, i.e. loses its planarity, thereby causing formationof steps. As a result, during movement of a polished semiconductor waferto the outer circumferential edge of the polishing pad in theaforementioned overhanging action, the semiconductor wafer may becracked or scratched because of the irregularities of the polishing pad.

DISCLOSURE OF INVENTION

The present invention has been made in view of the above drawbacks. Itis, therefore, an object of the present invention to provide a polishingmethod which can easily and safely separate a polished workpiece form apolishing surface without an overhanging action and can increasethroughput.

In order to attain the above object, according to a first aspect of thepresent invention, there is provided a polishing method. A workpiece anda polishing table are rotated. The polishing table has a polishingsurface thereon. The workpiece is pressed against the polishing surfaceon the polishing table rotated at a first rotational speed to polish theworkpiece. The workpiece is separated from the polishing surface afterthe workpiece is pressed against the polishing surface. Before theworkpiece is separated from the polishing surface, a rotational speed ofthe polishing table is reduced to a second rotational speed lower thanthe first rotational speed to provide a difference between a rotationalspeed of the workpiece and the rotational speed of the polishing table.

According to the first aspect of the present invention, the rotationalspeed of the polishing table is reduced from a rotational speed of thepolishing table rotated at the time of polishing to provide a differencebetween the rotational speeds of the polishing table and the workpiece.At that time, the workpiece floats on a liquid layer of polishing liquidwhich is formed between the lower surface of the workpiece and thepolishing surface. Specifically, a hydroplaning phenomenon occurs. Thehydroplaning phenomenon reduces surface tension between the polishingsurface and the workpiece due to the polishing liquid. Therefore, evenif the workpiece is lifted directly at a position where the polishingprocess is completed, the workpiece can readily be removed or separatedfrom the polishing surface without an overhangining action. Thus, theworkpiece is not left on the polishing surface and is prevented frombeing cracked or scratched, which would be caused by an overhangingaction. Further, since it is not necessary to perform an overhangingaction, polishing tact time can be shortened, and throughput can beimproved.

According to a preferred aspect of the present invention, the reductionof the rotational speed of the polishing table is performed during thepolishing process. When the rotational speed of the polishing table isreduced, a cleaning process of the polishing surface is performed. Inthis case, separation of the workpiece from the polishing surface isfacilitated, and conditions for the next workpiece can timely beadjusted. As a result, throughput is improved.

According to a preferred aspect of the present invention, a polishingliquid to the polishing surface is supplied at a first flow rate duringthe polishing process. Before or during the separation of the workpiecefrom the polishing surface, a flow rate of the polishing liquid isincreased to a second flow rate higher than the first flow rate. In thiscase, it is possible to enhance effects of a hydroplaning phenomenon andto easily separate the workpiece from the polishing surface.

According to a preferred aspect of the present invention, a mixture ofgas and pure water or chemical liquid is ejected to the polishingsurface to clean the polishing surface before the separation of theworkpiece from the polishing surface. In this case, it is possible toenhance effects of a hydroplaning phenomenon and to start a cleaningprocess of the polishing surface, which would be performed after thepolishing process, during the polishing process.

According to a preferred aspect of the present invention, the workpieceis lifted from the polishing surface to separate the workpiece form thepolishing surface. A lifting speed of the workpiece is reduced until theworkpiece has substantially been separated from the polishing surface.Thus, stresses caused to the workpiece when the workpiece is separatedfrom the polishing surface can be reduced.

According to a second aspect of the present invention, there is provideda polishing method. A workpiece and a polishing surface are movedrelative to each other. The workpiece is pressed against the polishingsurface moved at a first frequency to polish the workpiece. Theworkpiece is separated from the polishing surface after the workpiece ispressed against the polishing surface. Before the workpiece is separatedfrom the polishing surface, a frequency of movement of the polishingsurface is reduced to a second frequency lower than the first frequency.The workpiece and the polishing surface may make an orbital movement.

According to the second aspect of the present invention, the workpiecefloats on a liquid layer of polishing liquid which is formed between thelower surface of the workpiece and the polishing surface. Specifically,a hydroplaning phenomenon occurs. The hydroplaning phenomenon reducessurface tension between the polishing surface and the workpiece due tothe polishing liquid. Therefore, even if the workpiece islifted-directly at a position where the polishing process is completed,the workpiece can readily be removed or separated from the polishingsurface without an overhangining action.

According to a third aspect of the present invention, there is provideda polishing method. A polishing surface is moved relative to aworkpiece. The workpiece is pressed against the polishing surface movedat a first speed to polish the workpiece. The workpiece is separatedfrom the polishing surface after the workpiece is pressed against thepolishing surface. Before the workpiece is separated from the polishingsurface, a speed of movement of the polishing surface is reduced to asecond speed smaller than the first speed. The polishing surface may bemoved linearly.

According to the third aspect of the present invention, when the speedof movement of the polishing surface is reduced, a hydroplaningphenomenon occurs to reduce surface tension between the polishingsurface and the workpiece due to the polishing liquid. Therefore, evenif the workpiece is lifted directly at a position where the polishingprocess is completed, the workpiece can readily be removed from thepolishing surface without an overhangining action.

According to a fourth aspect of the present invention, there is provideda polishing method. A workpiece and a polishing table are rotated. Thepolishing table has a polishing surface thereon. The workpiece ispressed against the polishing surface on the polishing table. Theworkpiece is separated from the polishing surface after the workpiece ispressed against the polishing surface. A ratio of a rotational speed ofthe top ring to a rotational speed of the polishing table is reducedbefore the workpiece is separated from the polishing surface.

The above and other objects, features, and advantages of the presentinvention will be apparent from the following description when taken inconjunction with the accompanying drawings which illustrates preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is schematic view showing a basic arrangement of a polishingapparatus;

FIG. 2 is a front view showing a polishing apparatus employing apolishing method according to an embodiment of the present invention;

FIG. 3 is a plan view of the polishing apparatus shown in FIG. 2;

FIG. 4 is a cross-sectional view showing a top ring in the polishingapparatus shown in FIG. 2;

FIG. 5 is a bottom view of the top ring shown in FIG. 4;

FIG. 6 is a timing chart showing an example of a polishing process topolish a semiconductor wafer and a separating process to separate thesemiconductor wafer from a polishing surface according to an embodimentof the present invention; and

FIG. 7 is a perspective view showing a polishing apparatus having aweb-type (belt-type) polishing table which can employ the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

A polishing method according to embodiments of the present inventionwill be described below with reference to FIGS. 2 through 7.

FIG. 2 is a front view showing an arrangement of a polishing apparatusemploying a polishing method according to an embodiment of the presentinvention, and FIG. 3 is a plan view of the polishing apparatus shown inFIG. 2. As shown in FIGS. 2 and 3, the polishing apparatus has a topring 1 for holding a semiconductor wafer W and a polishing table 100disposed below the top ring 1. The polishing table 100 has a polishingpad 101 attached to an upper surface of the polishing table 100. Thepolishing pad 101 has an upper surface which is brought into slidingcontact with the semiconductor wafer W. Thus, the upper surface of thepolishing pad 101 serves as a polishing surface for polishing asemiconductor wafer W. The polishing apparatus also has a polishingliquid supply nozzle 102 disposed above the polishing table 100 forsupplying a polishing liquid Q onto the polishing pad 101, and anatomizer 103 having a plurality of ejection nozzles 103 a connected to anitrogen gas supply source and a liquid supply source. In FIG. 3, theatomizer 103 may be disposed at a position indicated by solid lines orat a position indicated by chain double-dashed lines.

The polishing liquid supply nozzle 102 supplies a polishing liquid Q,such as abrasive liquid or pure water, which is used for polishing thesemiconductor wafer W, onto the polishing surface on the polishing table100. The atomizer 103 ejects a mixture of nitrogen gas and pure water orchemical liquid to the polishing surface on the polishing table 100. Thenitrogen gas and the pure water or chemical liquid are regulated inpressures to predetermined values through regulators or air operatorvalves (not shown) connected to the nitrogen gas supply source and theliquid supply source and are then supplied to the ejection nozzles 103 aof the atomizer 103 in a mixed state. In this case, the ejection nozzles103 a of the atomizer 103 should preferably eject the mixture toward aperipheral edge of the polishing table 100. The atomizer 103 may employany inert gas other than nitrogen gas. The atomizer 103 may eject onlyliquid such as pure water or chemical liquid.

The mixture of the nitrogen gas and the pure water or chemical liquidmay be supplied in a state of (1) liquid fine particles, (2) solid fineparticles as a result of solidification of the liquid, or (3) gas as aresult of vaporization of the liquid. These states (1), (2), and (3) arereferred to as atomization. In these states, the mixture may be ejectedfrom the ejection nozzles 103 a of atomizer 103 toward the polishingsurface on the polishing table 100. For example, pressure or temperatureof the nitrogen gas and/or the pure water or chemical liquid, or theshape of the nozzles determines which state of the mixture is to beejected, i.e., liquid fine particles, solid fine particles, or gas.Therefore, the state of the mixture to be ejected can be varied, forexample, by properly adjusting pressure or temperature of the nitrogengas and/or the pure water or chemical liquid with use of a regulator orthe like, or by properly adjusting the shape of the nozzles.

Various kinds of polishing pads are available on the market. Forexample, some of these are SUBA800, IC-1000, and IC-1000/SUBA400(two-layer cloth) manufactured by Rodel Inc., and Surfin xxx-5 andSurfin 000 manufactured by Fujimi Inc. SUBA800, Surfin xxx-5, and Surfin000 are non-woven fabrics bonded by urethane resin, and IC-1000 is madeof rigid polyurethane foam (single-layer). Polyurethane foam is porousand has a large number of fine recesses or holes formed in its surface.

As shown in FIG. 2, the top ring 1 is connected to a top ring driveshaft 11 via a universal joint 10. The top ring drive shaft 11 iscoupled to a top ring air cylinder 111 fixed to a top ring head 110. Thetop ring air cylinder 111 operates to move the top ring drive shaft 11vertically to thereby lift and lower the top ring 1 as a whole and topress a retainer ring 3 fixed to a lower end of a top ring body 2against the polishing table 100. The top ring air cylinder 111 isconnected to a compressed air source 120 via a regulator R1, which canregulate pressure of compressed air or the like which is supplied to thetop ring air cylinder 111. Thus, it is possible to adjust a pressingforce to press the polishing pad 101 with the retainer ring 3.

The top ring drive shaft 11 is connected to a rotary sleeve 112 by a key(not shown). The rotary sleeve 112 has a timing pulley 113 fixedlydisposed at a peripheral portion thereof. A top ring motor 114 is fixedto the top ring head 110. The timing pulley 113 is coupled to a timingpulley 116 mounted on the top ring motor 114 via a timing belt 115.Therefore, when the top ring motor 114 is energized for rotation, therotary sleeve 112 and the top ring drive shaft 11 are rotated in unisonwith each other via the timing pulley 116, the timing belt 115, and thetiming pulley 113 to thereby rotate the top ring 1. The top ring head110 is supported on a top ring head shaft 117 fixedly supported on aframe (not shown).

The top ring 1 of the polishing apparatus will be described below. FIG.4 is a cross-sectional view showing the top ring 1 of the polishingapparatus, and FIG. 5 is a bottom view of the top ring 1 shown in FIG.4. As shown in FIG. 4, the top ring 1 has a top ring body 2 in the formof a cylindrical housing with a receiving space defined therein, and aretainer ring 3 fixed to the lower end of the top ring body 2. The topring body 2 is made of a material having high strength and rigidity,such as metal or ceramics. The retainer ring 3 is made of highly rigidsynthetic resin, ceramics, or the like.

The top ring body 2 has a cylindrical housing 2 a, an annularpressurizing sheet support 2 b fitted into the cylindrical portion ofthe housing 2 a, and an annular seal 2 c fitted over an outercircumferential edge of an upper surface of the housing 2 a. Theretainer ring 3 is fixed to the lower end of the housing 2 a of the topring body 2. The retainer ring 3 has a lower portion projecting radiallyinwardly. The retainer ring 3 may be formed integrally with the top ringbody 2.

The top ring drive shaft 11 is disposed above the central portion of thehousing 2 a of the top ring body 2. The top ring body 2 is coupled tothe top ring drive shaft 11 by the universal joint 10. The universaljoint 10 has a spherical bearing mechanism by which the top ring body 2and the top ring drive shaft 11 are tiltable with respect to each other,and a rotation transmitting mechanism for transmitting rotation of thetop ring drive shaft 11 to the top ring body 2. The spherical bearingmechanism and the rotation transmitting mechanism transmit a pressingforce and a rotating force from the top ring drive shaft 11 to the topring body 2 while allowing the top ring body 2 and the top ring driveshaft 11 to be tilted with respect to each other.

The spherical bearing mechanism has a hemispherical concave recess 11 adefined centrally in the lower surface of the top ring drive shaft 11, ahemispherical concave recess 2 d defined centrally in the upper surfaceof the housing 2 a, and a bearing ball 12 made of a highly hard materialsuch as ceramics and interposed between the concave recesses 11 a and 2d. The rotation transmitting mechanism includes drive pins (not shown)fixed to the top ring drive shaft 11 and driven pins (not shown) fixedto the housing 2 a. Even if the top ring body 2 is tilted with respectto the top ring drive shaft 11, the drive pins and the driven pinsremain in engagement with each other while contact points are displacedbecause the drive pins and the driven pins are vertically movablerelative to each other. Thus, the rotation transmitting mechanismreliably transmits rotational torque of the top ring drive shaft 11 tothe top ring body 2.

The top ring body 2 and the retainer ring 3 secured to the top ring body2 have a space defined therein, which accommodates therein a seal ring 4having a lower surface brought into contact with a peripheral portion ofthe semiconductor wafer W held by the top ring 1, an annular holder ring5, and a disk-shaped chucking plate 6 which is vertically movable withinthe receiving space in the top ring body 2. The seal ring 4 has aradially outer edge clamped between the holder ring 5 and the chuckingplate 6 secured to the lower end of the holder ring 5 and extendsradially inwardly so as to cover the lower surface of the chucking plate6 near its outer circumferential edge. The lower end surface of the sealring 4 is brought into contact with the upper surface of thesemiconductor wafer W to be polished. The semiconductor wafer W has arecess defined in an outer edge thereof, which is referred to as a notchor orientation flat, for recognizing (identifying) orientation of thesemiconductor wafer. The seal ring 4 should preferably extend radiallyinwardly of the chucking plate 6 from the innermost position of such asa notch or orientation flat.

The chucking plate 6 may be made of metal. However, when the thicknessof a thin film formed on a surface of a semiconductor wafer is measuredby a method using eddy current in a state such that the semiconductorwafer to be polished is held by the top ring, the chucking plate 6should preferably be made of a non-magnetic material, e.g. an insulatingmaterial such as fluororesin, epoxy resin, or ceramics.

A pressurizing sheet 7 comprising an elastic membrane extends betweenthe holder ring 5 and the top ring body 2. The pressurizing sheet 7 hasa radially outer edge clamped between the housing 2 a and thepressurizing sheet support 2 b of the top ring body 2, and a radiallyinner edge clamped between an upper end portion 5 a and a stopper 5 b ofthe holder ring 5. The top ring body 2, the chucking plate 6, the holderring 5, and the pressurizing sheet 7 jointly define a pressure chamber21 in the top ring body 2. As shown in FIG. 4, a fluid passage 31comprising tubes and connectors communicates with the pressure chamber21, which is connected to the compressed air source 120 via a regulatorR2 provided on the fluid passage 31. The pressurizing sheet 7 is made ofa highly strong and durable rubber material such as ethylene propylenerubber (EPDM), polyurethane rubber, or silicone rubber.

In a case where the pressurizing sheet 7 is made of an elastic materialsuch as rubber, if the pressurizing sheet 7 is fixedly clamped betweenthe retainer ring 3 and the top ring body 2, then a desired horizontalsurface cannot be maintained on the lower surface of the retainer ring 3because of elastic deformation of the pressurizing sheet 7 as an elasticmaterial. In order to prevent such a drawback, the pressurizing sheet 7is clamped between the housing 2 a of the top ring body 2 and thepressurizing sheet support 2 b provided as a separate member in thepresent embodiment. The retainer ring 3 may be movable vertically withrespect to the top ring body 2, or the retainer ring 3 may have astructure capable of pressing the polishing surface independently of thetop ring body 2. In such cases, the pressurizing sheet 7 is notnecessarily fixed in the aforementioned manner.

A cleaning liquid passage 51 in the form of an annular groove is definedin the upper surface of the housing 2 a near its outer circumferentialedge over which the seal 2 c of the top ring body 2 is fitted. Thecleaning liquid passage 51 communicates with a fluid passage 32 througha through-hole 52 formed in the seal 2 c. A cleaning liquid (pure water)is supplied through the fluid passage 32 to the cleaning liquid passage51. A plurality of communication holes 53 are defined in the housing 2 aand the pressurizing sheet support 2 b in communication with thecleaning liquid passage 51. The communication holes 53 communicate witha small gap G defined between the outer circumferential surface of theseal ring 4 and the inner circumferential surface of the retainer ring3.

A central bag 8 and a ring tube 9 which serve as abutment membersbrought into contact with the semiconductor wafer W are mounted in aspace defined between the chucking plate 6 and the semiconductor waferW. In the present embodiment, as shown in FIGS. 4 and 5, the central bag8 is disposed centrally on the lower surface of the chucking plate 6,and the ring tube 9 is disposed radially outwardly of the central bag 8in surrounding relation thereto. Each of the seal ring 4, the centralbag 8, and the ring tube 9 is made of a highly strong and durable rubbermaterial such as ethylene propylene rubber (EPDM), polyurethane rubber,or silicone rubber.

The space defined between the chucking plate 6 and the semiconductorwafer W is divided into a plurality of spaces by the central bag 8 andthe ring tube 9. Accordingly, a pressure chamber 22 is defined betweenthe central bag 8 and the ring tube 9, and a pressure chamber 23 isdefined radially outwardly of the ring tube 9.

The central bag 8 has an elastic membrane 81 brought into contact withthe upper surface of the semiconductor wafer W, and a central bag holder82 for detachably holding the elastic membrane 81 in position. Thecentral bag holder 82 has threaded holes 82 a defined therein, and thecentral bag 8 is detachably fastened to the center of the lower surfaceof the chucking plate 6 by screws 55 threaded into the threaded holes 82a. The central bag 8 has a central pressure chamber 24 defined thereinby the elastic membrane 81 and the central bag holder 82.

Similarly, the ring tube 9 has an elastic membrane 91 brought intocontact with the upper surface of the semiconductor wafer W, and a ringtube holder 92 for detachably holding the elastic membrane 91 inposition. The ring tube holder 92 has threaded holes 92 a definedtherein, and the ring tube 9 is detachably fastened to the lower surfaceof the chucking plate 6 by screws 56 threaded into the threaded holes 92a. The ring tube 9 has an intermediate pressure chamber 25 definedtherein by the elastic membrane 91 and the ring tube holder 92.

In the present embodiment, the pressure chamber 24 is formed by theelastic membrane 81 of the central bag 8 and the central bag holder 82,and the pressure chamber 25 is formed by the elastic membrane 91 of thering tube 9 and the ring tube holder 92. The pressure chambers 22 and 23may also be formed by an elastic membrane and a holder for fixing theelastic membrane, respectively. Further, elastic membranes and holdersmay appropriately be added to increase the number of the pressurechambers.

Fluid passages 33, 34, 35 and 36 comprising tubes and connectorscommunicate with the pressure chambers 22 and 23, the central pressurechamber 24, and the intermediate pressure chamber 25, respectively. Thepressure chambers 22 to 25 are connected to the compressed air source120 as a supply source via respective regulators R3, R4, R5 and R6connected respectively to the fluid passages 33 to 36. The fluidpassages 31 to 36 are connected to the respective regulators R1 to R6through a rotary joint (not shown) mounted on the upper end of the topring shaft 11.

The pressure chamber 21 above the chucking plate 6 and the pressurechambers 22 to 25 are supplied with pressurized fluids such aspressurized air or atmospheric air or evacuated, via the fluid passages31, 33, 34, 35 and 36 connected to the respective pressure chambers. Asshown in FIG. 2, the regulators R2 to R6 connected to the fluid passages31, 33, 34, 35 and 36 of the pressure chambers 21 to 25 can respectivelyregulate the pressures of the pressurized fluids supplied to therespective pressure chambers. Thus, it is possible to independentlycontrol the pressures in the pressure chambers 21 to 25 or independentlyintroduce atmospheric air or vacuum into the pressure chambers 21 to 25.In this manner, the pressures in the pressure chambers 21 to 25 areindependently varied with the regulators R2 to R6, so that the pressingforces to press the semiconductor wafer W against the polishing pad 101can be adjusted in local areas of the semiconductor wafer W. In someapplications, the pressure chambers 21 to 25 may be connected to avacuum source 121.

In this case, the pressurized fluid or the atmospheric air supplied tothe pressure chambers 22 to 25 may independently be controlled intemperature. With this configuration, it is possible to directly controlthe temperature of a workpiece such as a semiconductor wafer from thebackside of the surface to be polished. Particularly, when each of thepressure chambers is independently controlled in temperature, the rateof chemical reaction can be controlled in the chemical polishing processof CMP.

The chucking plate 6 has radially inner suction portions 61 extendeddownwardly therefrom between the central bag 8 and the ring tube 9. Thechucking plate 6 has radially outer suction portions 62 extendeddownwardly therefrom outside of the ring tube 9. In the presentembodiment, eight suction portions 61, 62 are provided.

The inner suction portions 61 and the outer suction portions 62 havecommunication holes 61 a, 62 a communicating with fluid passages 37, 38,respectively. The inner suction portions 61 and the outer suctionportions 62 are connected to the vacuum source 121 such as a vacuum pumpvia the fluid passages 37, 38 and valves V1, V2. When the communicationholes 61 a, 62 a of the suction portions 61, 62 are connected to thevacuum source 121, a negative pressure is developed at the lower openingends of the communication holes 61 a, 62 a thereof to attract asemiconductor wafer W to the lower ends of the inner suction portions 61and the outer suction portions 62. The inner suction portions 61 and theouter suction portions 62 have elastic sheets 61 b, 62 b, such as thinrubber sheets, attached to their lower ends, for thereby elasticallycontacting and holding the semiconductor wafer W on the lower surfacesthereof.

Since there is a small gap G between the outer circumferential surfaceof the seal ring 4 and the inner circumferential surface of the retainerring 3, the holder ring 5, the chucking plate 6, and the seal ring 4attached to the chucking plate 6 can vertically be moved with respect tothe top ring body 2 and the retainer ring 3, and hence are of a floatingstructure with respect to the top ring body 2 and the retainer ring 3.The stopper 5 b of the holder ring 5 has a plurality of teeth 5 cprojecting radially outwardly from the outer circumferential edgethereof. Downward movement of members including the holder ring 5 islimited to a predetermined range by engaging the teeth 5 c with theupper surface of the radially inwardly projecting portion of theretainer ring 3.

Operation of the top ring 1 thus constructed will be described in detailbelow.

In the polishing apparatus thus constructed, when a semiconductor waferW is to be delivered to the polishing apparatus, the top ring 1 as awhole is moved to a position to which the semiconductor wafer W istransferred, and the communication holes 61 a and 62 a of the suctionportions 61 and 62 are connected through the fluid passages 37 and 38 tothe vacuum source 121. The semiconductor wafer W is attracted undervacuum to the lower ends of the suction portions 61 and 62 by suctioneffect of the communication holes 61 a and 62 a. With the semiconductorwafer W attracted to the top ring 1, the top ring 1 as a whole is movedto a position above the polishing table 100 having the polishing surface(polishing pad 101) thereon. The outer circumferential edge of thesemiconductor wafer W is held by the retainer ring 3 so that thesemiconductor wafer W is not dislodged from the top ring 1.

For polishing the semiconductor wafer W, the attraction of semiconductorwafer W by the suction portions 61 and 62 is released, and thesemiconductor wafer W is held on the lower surface of the top ring 1.Simultaneously, the top ring air cylinder 111 connected to the top ringdrive shaft 11 is actuated to press the retainer ring 3 fixed to thelower end of the top ring 1 against the polishing surface on thepolishing table 100 under a predetermined pressure. In such a state,pressurized fluids are respectively supplied to the pressure chambers22, 23, the central pressure chamber 24, and the intermediate pressurechamber 25 under respective pressures, thereby pressing thesemiconductor wafer W against the polishing surface on the polishingtable 100. The polishing liquid supply nozzle 102 supplies a polishingliquid Q onto the polishing pad 101 in advance, so that the polishingliquid Q is held on the polishing pad 101. Thus, the semiconductor waferW is polished by the polishing pad 101 with the polishing liquid Q beingpresent between the (lower) surface of the semiconductor wafer W to bepolished and the polishing pad 101. At the time of polishing, therotational speed of the polishing table 100 is maintained to besubstantially the same as the rotational speed of the top ring 1.

The local areas of the semiconductor wafer W that are positioned beneaththe pressure chambers 22 and 23 are pressed against the polishingsurface under the pressures of the pressurized fluids supplied to thepressure chambers 22 and 23. The local area of the semiconductor wafer Wthat is positioned beneath the central pressure chamber 24 is pressedvia the elastic membrane 81 of the central bag 8 against the polishingsurface under the pressure of the pressurized fluid supplied to thecentral pressure chamber 24. The local area of the semiconductor wafer Wthat is positioned beneath the intermediate pressure chamber 25 ispressed via the elastic membrane 91 of the ring tube 9 against thepolishing surface under the pressure of the pressurized fluid suppliedto the intermediate pressure chamber 25.

Therefore, the polishing pressures acting on the respective local areasof the semiconductor wafer W can be adjusted independently bycontrolling the pressures of the pressurized fluids supplied to therespective pressure chambers 22 to 25. Specifically, the respectiveregulators R3 to R6 independently regulate the pressures of thepressurized fluids supplied to the pressure chambers 22 to 25 to therebyadjust the pressing forces applied to press the local areas of thesemiconductor wafer W against the polishing pad 101 on the polishingtable 100. With the polishing pressures on the respective local areas ofthe semiconductor wafer W being adjusted independently to desiredvalues, the semiconductor wafer W is pressed against the polishing pad101 on the polishing table 100 that is being rotated. Similarly, thepressure of the pressurized fluid supplied to the top ring air cylinder111 can be regulated by the regulator R1 to adjust the force with whichthe retainer ring 3 presses the polishing pad 101. While thesemiconductor wafer W is being polished, the force with which theretainer ring 3 presses the polishing pad 101 and the pressing forcewith which the semiconductor wafer W is pressed against the polishingpad 101 can appropriately be adjusted to thereby apply polishingpressures in a desired pressure distribution to a central area (C1 inFIG. 5), an inner area (C2) between the central area and an intermediatearea, the intermediate area (C3), a peripheral area (C4) of thesemiconductor wafer W, and a peripheral portion of the retainer ring 3which is positioned outside of the semiconductor wafer W.

In this manner, the semiconductor wafer W is divided into the fourconcentric circular and annular areas (C1 to C4), which can respectivelybe pressed under independent pressing forces. A polishing rate dependson a pressing force applied to a semiconductor wafer W against apolishing surface. As described above, since the pressing forces appliedto those areas can independently be controlled, the polishing rates ofthe four circular and annular areas (C1 to C4) of the semiconductorwafer W can independently be controlled. Consequently, even if thethickness of a thin film to be polished on the surface of thesemiconductor wafer W suffers radial variations, the thin film on thesurface of the semiconductor wafer W can be polished uniformly withoutbeing insufficiently or excessively polished over the entire surface ofthe semiconductor wafer. More specifically, even if the thickness of thethin film to be polished on the surface of the semiconductor wafer Wdiffers depending on the radial position on the semiconductor wafer W,the pressure in a pressure chamber positioned over a thicker area of thethin film is made higher than the pressure in other pressure chambers,or the pressure in a pressure chamber positioned over a thinner area ofthe thin film is made lower than the pressure in other pressurechambers. In this manner, the pressing force applied to the thicker areaof the thin film against the polishing surface is made higher than thepressing force applied to the thinner area of the thin film against thepolishing surface, thereby selectively increasing the polishing rate ofthe thicker area of the thin film. Consequently, the entire surface ofthe semiconductor wafer W can be polished exactly to a desired levelover the entire surface of the semiconductor wafer W irrespective of thefilm thickness distribution produced at the time when the thin film isformed.

Any unwanted edge rounding on the circumferential edge of thesemiconductor wafer W can be prevented by controlling the pressing forceapplied to the retainer ring 3. If the thin film to be polished on thecircumferential edge of the semiconductor wafer W has large thicknessvariations, then the pressing force applied to the retainer ring 3 isintentionally increased or reduced to thus control the polishing rate ofthe circumferential edge of the semiconductor wafer W. When thepressurized fluids are supplied to the pressure chambers 22 to 25, thechucking plate 6 is subjected to upward forces. In the presentembodiment, the pressurized fluid is supplied to the pressure chamber 21through the fluid passage 31 to prevent the chucking plate 6 from beinglifted under the forces due to the pressure chambers 22 to 25.

As described above, the pressing force applied by the top ring aircylinder 111 to press the retainer ring 3 against the polishing pad 101and the pressing forces applied by the pressurized air supplied to thepressure chambers 22 to 25 to press the local areas of the semiconductorwafer W against the polishing pad 101 are appropriately adjusted topolish the semiconductor wafer W. When the polishing of thesemiconductor wafer W is finished, the semiconductor wafer W isattracted to the lower ends of the inner suction portions 61 and theouter suction portions 62 under vacuum in the same manner as describedabove. At that time, the supply of the pressurized fluids into thepressure chambers 22 to 25 to press the semiconductor wafer W againstthe polishing surface is stopped, and the pressure chambers 22 to 25 arevented to the atmosphere. Accordingly, the lower ends of the suctionportions 61 and 62 are brought into contact with the semiconductor waferW. The pressure chamber 21 is vented to the atmosphere or evacuated todevelop a negative pressure therein. If the pressure chamber 21 ismaintained at a high pressure, then the semiconductor wafer W isstrongly pressed against the polishing surface only in areas broughtinto contact with the suction portions 61 and 62. Therefore, it isnecessary to decrease the pressure in the pressure chamber 21immediately. Accordingly, a relief port 39 penetrating from the pressurechamber 21 through the top ring body 2 may be provided for decreasingthe pressure in the pressure chamber 21 immediately, as shown in FIG. 4.In this case, when the pressure chamber 21 is pressurized, it isnecessary to continuously supply the pressurized fluid into the pressurechamber 21 via the fluid passage 31. The relief port 39 comprises acheck valve for preventing an outside air from flowing into the pressurechamber 21 at the time when a negative pressure is developed in thepressure chamber 21.

After the semiconductor wafer W is thus attracted under suction to thesuction portions 61 and 62 as described above, it is necessary to remove(or separate) the semiconductor wafer W from the polishing pad 101. Inthis case, a large surface tension adversely acts between the polishingpad 101 and the semiconductor wafer W as described above. According tothe present invention, in order to reduce surface tension between thepolishing pad 101 and the semiconductor wafer W, the rotational speed ofthe polishing table 100 is reduced from a rotational speed: of thepolishing table 100 rotated at the time of polishing. In this case, therotational speed of the top ring 1 is maintained at the same rotationalspeed as a rotational speed of the top ring 1 rotated at the time ofpolishing.

Specifically, the rotational speed of the polishing table 100 is reducedfrom a rotational speed of the polishing table 100 rotated at the timeof polishing to provide a difference between the rotational speeds ofthe polishing table 100 and the top ring 1. At that time, thesemiconductor wafer W held by the top ring 1 floats on a liquid layer ofthe polishing liquid Q which is formed between the lower surface of thesemiconductor wafer W and the upper surface of the polishing pad 101.This phenomenon is referred to as a hydroplaning phenomenon. Thehydroplaning phenomenon reduces surface tension between the polishingsurface and the semiconductor wafer W due to the polishing liquid.Therefore, even if the top ring 1 is lifted directly at a position wherethe polishing process is completed, the semiconductor wafer W canreadily be removed or separated from the polishing pad 101 without anoverhangining action. Thus, the semiconductor wafer W is not left on thepolishing pad 101 and is prevented from being cracked or scratched,which would be caused by an overhanging action.

When the top ring 1 is lifted to separate the semiconductor wafer W fromthe polishing surface, the speed of lifting the top ring 1 is maderelatively low until the semiconductor wafer W has substantially beenseparated from the polishing surface. After the semiconductor wafer Whas been separated from the polishing surface, the speed of lifting thetop ring 1 is increased. Specifically, the top ring 1 is lifted at a lowspeed when the semiconductor wafer W is separated from the polishingsurface and at a high speed after the semiconductor wafer W has beenseparated from the polishing surface. Thus, pressure applied to thesemiconductor wafer W when the semiconductor wafer W is separated fromthe polishing surface is made as small as possible to minimize stressesapplied to circuit elements formed on the semiconductor wafer W. Thelifting speed of the top ring 1 can be controlled by adjusting pressureof compressed air to be supplied to the top ring air cylinder 111. Forexample, a sensor (not shown) may be provided near a rod of the top ringair cylinder 111 for detecting a position of the semiconductor wafer Wwhen the semiconductor wafer W has substantially been separated from thepolishing surface to control the lifting speed of the top ring 1.

According to the present invention, since it is not necessary to performan overhanging action, polishing tact time can be shortened, andthroughput can be improved.

It is desirable that the rotational speed of the polishing table 100 isreduced to provide a difference between the rotational speeds of the topring 1 and the polishing table 100 during the polishing process andbefore the semiconductor wafer W is separated from the polishingsurface, and that the top ring 1 is lifted as it is when the polishingprocess has been completed. Specifically, the rotational speed of thepolishing table 100 is reduced immediately before the polishing processhas been completed to provide a difference between the rotational speedsof the top ring 1 and the polishing table 100. The polishing process iscontinued in this state. When the polishing process is completed, thetop ring 1 is lifted as it is to separate the semiconductor wafer W fromthe polishing surface. According to this method, preparation forseparating the semiconductor wafer W from the polishing surface can bestarted during the polishing process. Therefore, it is possible toshorten polishing tact time and improve throughput.

It is desirable that the amount of polishing liquid Q supplied from thepolishing liquid supply nozzle 102 is increased before or during theseparation of the semiconductor wafer W from the polishing surface. Insuch a case, it is possible to enhance effects of a hydroplaningphenomenon and to easily separate the semiconductor W from the polishingsurface. The atomizer 103 should preferably start to eject a mixture ofgas (e.g. inert gas such as N₂ gas) and pure water or a chemical liquidfor cleaning the polishing surface toward the polishing surfaceimmediately before the semiconductor wafer W is separated from thepolishing surface. Thus, it is possible to enhance effects of ahydroplaning phenomenon and to start a cleaning process of the polishingsurface, which would be performed after the polishing process, duringthe polishing process.

After the top ring 1 has been lifted as described above, the top ring 1is moved to a position at which the semiconductor wafer is transferred.Then, a fluid (e.g. compressed air or a mixture of nitrogen and purewater) is ejected through the communication holes 61 a and 62 b of thesuction portions 61 and 62 to the semiconductor wafer W to release thesemiconductor wafer W.

The polishing liquid Q used to polish the semiconductor wafer W tends toflow through the small gap G between the outer circumferential surfaceof the seal ring 4 and the retainer ring 3. If the polishing liquid Q isfirmly deposited in the gap G, then the holder ring 5, the chuckingplate 6, and the seal ring 4 are prevented from smoothly movingvertically with respect to the top ring body 2 and the retainer ring 3.To avoid such a drawback, a cleaning liquid (pure water) is suppliedthrough the fluid passage 32 to the cleaning liquid passage 51.Accordingly, the pure water is supplied through a plurality ofcommunication holes 53 to a region above the gap G, thus cleaning thegap G to prevent the polishing liquid Q from being firmly deposited inthe gap G. The pure water should preferably be supplied after thepolished semiconductor wafer W is released and until the nextsemiconductor wafer to be polished is attracted to the top ring 1. It isdesirable to discharge all the supplied pure water out of the top ring 1before the next semiconductor wafer is polished, and hence to providethe retainer ring 3 with a plurality of through-holes 3 a shown in FIG.4. Furthermore, if a pressure buildup is developed in a space 26 definedbetween the retainer ring 3, the holder ring 5, and the pressurizingsheet 7, then it acts to prevent the chucking plate 6 from beingelevated in the top ring body 2. Therefore, in order to allow thechucking plate 6 to be elevated smoothly in the top ring body 2, thethrough-holes 3 a should preferably be provided for equalizing thepressure in the space 26 with the atmospheric pressure.

In the present embodiment, as shown in FIG. 2, the fluid passages 37 and38 as a vacuum line communicating with the vacuum evacuation source 121is connected to the top ring 2. Thus, the semiconductor wafer W is heldon the lower surface of the top ring 2 by vacuum suction. When thesemiconductor wafer W is to be separated from the polishing surface, thetop ring 1 is mechanically lifted. If the semiconductor wafer W isdislodged from the top ring 1 and is left on the polishing pad 101, apressure of the vacuum line (fluid passages 37 and 38) changes to avacuum nearer to atmospheric pressure. Therefore, pressure of the vacuumline is monitored to judge whether or not the semiconductor wafer W hasnormally been separated from the polishing pad 101.

In the present embodiment, as shown in FIG. 2, a vacuum pressure sensor40 is provided in the vacuum line to measure and monitor pressure of thevacuum line, to thereby judge whether or not the semiconductor wafer Whas normally been separated from the polishing pad 101. Specifically,after a lifting operation of the top ring 1 is started, pressure of thevacuum line is measured with the vacuum pressure sensor 40. If themeasured value is less than a predetermined pressure value, it is judgedthat the semiconductor wafer W has normally been separated from thepolishing pad 101 in such a state that the semiconductor wafer W isattracted to the top ring 1. More specifically, if pressures that are atleast 10 kPa larger than the predetermined pressure value, at which thesemiconductor wafer W is judged to be normally attracted to the top ring1, are measured for a predetermined period or longer, then it is judgedthat the semiconductor wafer W has been dislodged from the top ring 1.

FIG. 6 is a timing chart showing an example of a polishing process topolish a semiconductor wafer and a separating process to separate thesemiconductor wafer from the polishing surface in the presentembodiment. With reference to FIG. 6, there will be described processesincluding rotation of the top ring, stop of the rotation of the topring, rotation of the polishing table, reduction of the rotational speedof the polishing table, stop of the rotation of the polishing table,supply of the polishing liquid from the polishing liquid supply nozzle,stop of the supply of the polishing liquid, supply of a mixture ofnitrogen gas and pure water or chemical liquid from the atomizer, andstop of the supply from the atomizer.

A polishing process is continued until T2. For example, the rotationalspeed of the top ring 1 is 71 rpm (71 min⁻¹), and the rotational speedof polishing table 100 is 70 rpm (70 min⁻¹). A polishing liquid issupplied from the polishing liquid supply nozzle 102 at a flow rate of150 ml/min. The atomizer 103 does not supply a mixture of gas and purewater or chemical liquid at that time.

Conditions are changed between T1 and T2 before the top ring 1 is lifted(i.e. the semiconductor wafer is separated from the polishing surface).At that time, the rotational speed of the top ring is 71 rpm (71 min⁻¹),which is the same as the rotational speed of the top ring rotated at thetime of the polishing process. The rotational speed of the polishingtable 100 begins to be reduced from 70 rpm (70 min⁻¹) at T1 and reaches25 rpm (25 min⁻¹) in a short period of time after T1. Then, thepolishing table 100 maintains a rotational speed of 25 rpm (25 min⁻¹).The flow rate of polishing liquid supplied from the polishing liquidsupply nozzle 102 begins to be increased at T1 and reaches 300 ml/min ina short period of time after T1. Then, the polishing liquid supplynozzle 102 maintains a flow rate of 300 ml/min. The atomizer 103 beginsto eject a mixture of nitrogen gas and pure water or chemical liquidonto the polishing surface at T1.

The top ring 1 begins to be lifted at T2 to separate the semiconductorwafer W from the polishing surface. The lifting operation of the topring 1 is completed at T3. At that time, the semiconductor wafer W hasbeen separated from the polishing surface by a predetermined distance.

The top ring 1 begins to stop its rotation at T4, and the polishingtable 100 also begins to stop its rotation at T4. The rotations of thetop ring 1 and the polishing table 100 are completely stopped at T′. Atthat time, the supply of the polishing liquid is also completelystopped. The supply of the mixture of nitrogen gas and pure water orchemical liquid from the atomizer 103 is continued until T5 and stoppedat T5. The atomizer usually begins to supply the mixture at T′ when therotations of the top ring and the polishing table are completely stoppedand continues to supply the mixture for a while after T5 as indicated bychain double-dashed lines.

As described above, according to the present embodiment, the atomizer103 begins to eject the mixture before the top ring is lifted.Therefore, the atomizer 103 can complete a cleaning process of thepolishing surface earlier as compared to a conventional method. Thus,conditions for the next workpiece can timely be adjusted, and hencethroughput is improved.

In the above example, the amount of polishing liquid supplied from thepolishing liquid supply nozzle 102 is increased at T1. In this case, acleaning liquid (pure water) for cleaning the retainer ring 3 may besupplied in synchronism with the increase of the amount of polishingliquid supplied from the polishing liquid supply nozzle 102.

The present invention is applicable not only to a polishing apparatushaving a rotatable polishing table as shown in FIGS. 2 and 3, but alsoto a polishing apparatus having a scroll-type polishing table or aweb-type (belt-type) polishing table.

A scroll-type polishing table has a diameter considerably smaller thanthe diameter of the polishing table 100 shown in FIGS. 2 and 3.Specifically, a scroll-type polishing table has a diameter equal to orlarger than the sum of the diameter of a semiconductor wafer and atwofold eccentric distance. Thus, the semiconductor is moved within thepolishing table even if the polishing table makes a translationalmovement (scrolling movement or orbital movement) with a radius of theeccentric distance. The scroll-type polishing table makes atranslational orbital movement (scrolling movement) at a predeterminedfrequency by actuation of a motor. In a polishing apparatus having ascroll-type polishing table, a semiconductor wafer is held and pressedagainst a polishing surface on the scroll-type polishing table by thesame top ring as that shown in FIGS. 2 and 4. A polishing liquid issupplied through holes defined in the polishing table to the polishingsurface. The polishing apparatus provides a relative movement betweenthe polishing surface and the semiconductor wafer to make a smalltranslational orbital movement with a radius of an eccentric distance.The semiconductor wafer is uniformly polished over the entire surfacethereof. If the polishing apparatus continuously has the same positionalrelationship between the surface of the semiconductor wafer and thepolishing surface, a polished surface of the semiconductor wafer may beinfluenced by local differences of the polishing surface. Therefore, thetop ring is gradually rotated to prevent the surface of thesemiconductor wafer from being polished only by the same portion of thepolishing surface.

When the scroll-type polishing table is used in a polishing apparatus, asemiconductor wafer is separated from the polishing surface as follows.Before the semiconductor wafer is separated from the polishing surface,the frequency of the orbital movement of the polishing table is reduced.When the polishing process is completed, the top ring is lifted as itis. Thus, the semiconductor wafer held by the top ring floats on aliquid layer of the polishing liquid which is formed between the lowersurface of the semiconductor wafer and the polishing surface.Specifically, a hydroplaning phenomenon occurs. Accordingly, surfacetension due to the polishing liquid is reduced between the polishingsurface and the semiconductor wafer. Therefore, even if the top ring islifted directly at a position where the polishing process is completed,the semiconductor wafer can readily be removed or separated from thepolishing surface without an overhangining action.

FIG. 7 is a perspective view showing a polishing apparatus having aweb-type (belt-type) polishing table. As shown in FIG. 7, the web-typepolishing table has a belt 215 having abrasive particles attached on asurface of the belt 215, a pair of rotatable drums 216 and 217 spacedfrom each other, and a supporting table 218 disposed between an upperbelt surface 215 a and a lower belt surface 215 b. The belt 215 is woundon the rotatable drums 216 and 217. When the rotatable drums 216 and 217are rotated, the belt 215 is linearly moved in a direction indicated byan arrow A. The polishing apparatus has the same top ring 1 as thatshown in FIGS. 2 and 4. A semiconductor wafer held by the top ring 1 ispressed against an upper surface of the belt 215 (i.e., polishingsurface). A polishing liquid Q is supplied from a polishing liquidsupply nozzle 102 onto the upper surface of the belt 215 to polish thesemiconductor wafer.

With the polishing apparatus shown in FIG. 7, the semiconductor wafer isseparated from the polishing surface as follows. Before thesemiconductor wafer is separated from the polishing surface, the speedof movement of the belt 215 is reduced. When the polishing process iscompleted, the top ring is lifted as it is. Thus, the semiconductorwafer held by the top ring 1 floats on a liquid layer of the polishingliquid which is formed between the lower surface of the semiconductorwafer and the upper surface of the belt 215. Specifically, ahydroplaning phenomenon occurs. Accordingly, surface tension due to thepolishing liquid is reduced between the polishing surface and thesemiconductor wafer. Therefore, even if the top ring is lifted directlyat a position where the polishing process is completed, thesemiconductor wafer can readily be removed or separated from thepolishing surface without an overhangining action. The amount ofpolishing liquid supplied from the polishing liquid supply nozzle 102may be increased before or during the separation of the semiconductorwafer from the polishing surface.

In the embodiment described above, the polishing surface is formed bythe polishing pad. However, the polishing surface is not limited to thepolishing pad. For example, the polishing surface may be formed by afixed abrasive. The fixed abrasive is formed into a flat platecomprising abrasive particles fixed by a binder. With the fixedabrasive, a polishing process is performed by abrasive particles thatare self-generated from the fixed abrasive. The fixed abrasive comprisesabrasive particles, a binder, and pores. For example, cerium dioxide(CeO₂) having an average particle diameter of 0.5 μm or less is used asan abrasive particle, and epoxy resin is used as a binder. Such a fixedabrasive forms a harder polishing surface. The fixed abrasive includes afixed abrasive pad having a two-layer structure formed by a thin layerof a fixed abrasive and an elastic polishing pad attached to a lowersurface of the thin layer of the fixed abrasive.

With the polishing apparatus shown in FIGS. 2 through 5, stresses causedto a semiconductor wafer were measured. Strain gauges were attached at acentral portion and a peripheral portion of a semiconductor wafer heldby the top ring. Rotational speeds of the polishing table and the topring were varied when the semiconductor wafer was separated from thepolishing surface. At that time, stresses caused to the semiconductorwafer were measured for several examples under the followingexperimental conditions. The measured results are shown in Table 1below.

1. A semiconductor wafer having a diameter of 300 mm was used.

2. The semiconductor wafer was separated from the polishing surfaceunder the following conditions.

EXAMPLE 1

The rotational speed of the polishing table was maintained at 60 rpm (60min⁻¹), and the rotational speed of the top ring was maintained at 60rpm (60 min⁻¹). These conditions are the same as those in a conventionalmethod. The top ring was lifted for 0.5 second.

EXAMPLE 2

The rotational speed of the polishing table was increased from 60 rpm(60 min⁻¹) to 70 rpm (70 min⁻¹), and the rotational speed of the topring was increased from 60 rpm (60 min⁻¹) to 71 rpm (71 min⁻¹). The topring was lifted for 0.5 second.

EXAMPLE 3

The rotational speed of the polishing table was increased from 70 rpm(70 min⁻¹) to 120 rpm (120 min⁻¹), and the rotational speed of the topring was increased to 120 rpm (120 min⁻¹). The top ring was lifted for0.5 second.

EXAMPLE 4

The rotational speed of the polishing table was reduced from 70 rpm (70min⁻¹) to 25 rpm (25 min⁻¹), and the rotational speed of the top ringwas maintained at 71 rpm (71 min⁻¹). The top ring was lifted for 1.5seconds.

EXAMPLE 5

The rotational speed of the polishing table was reduced from 70 rpm (70min⁻¹) to 25 rpm (25 min⁻¹), and the rotational speed of the top ringwas maintained at 71 rpm (71 min⁻¹). The top ring was lifted for 0.5second.

EXAMPLE 6

The rotational speed of the polishing table was reduced from 70 rpm (70min⁻¹) to 25 rpm (25 min⁻¹), and the rotational speed of the top ringwas reduced from 70 rpm (70 min⁻¹) to 50 rpm (50 min⁻¹). The top ringwas lifted for 0.5 second.

EXAMPLE 7

The rotational speed of the polishing table was reduced from 70 rpm (70min⁻¹) to 25 rpm (25 min⁻¹), and the rotational speed of the top ringwas reduced from 70 rpm (70 min⁻¹) to 40 rpm (40 min⁻¹). The top ringwas lifted for 0.5 second.

EXAMPLE 8

The rotational speed of the polishing table was reduced from 70 rpm (70min⁻¹) to 50 rpm (50 min⁻¹), and the rotational speed of the top ringwas maintained at 71 rpm (71 min⁻¹). The top ring was lifted for 0.5second. TABLE 1 Time Major Principal Stress [MPa] Level of Rotational toPeripheral Stress Speed of Rotational Lift Central Portion Portion inPolishing Speed of Top Strain [μ] Mea- Mea- Wafer Example Table Top RingRing Central Portion Peripheral Portion sured Average sured Average (1to No. [min⁻¹] [min⁻¹] [s] ε^(a) ε^(b) ε^(c) ε^(a) ε^(b) ε^(c) ValueValue Value Value 11) Note 1 60 60 0.5 −65.8 −18.7 −36.4 23.4 2.0 40.6−13.0 −12.4 9.1 8.3 5 Conventional −69.1 −39.4 −34.3 20.1 29.6 46.5−11.7 7.6 Method 2 70 71 0.5 −28.8 −32.1 −36.1 195.2 181.5 153.9 −6.4−7.9 34.6 38.0 8 −40.7 −54.3 −48.3 292.9 48.9 −47.8 −9.3 41.3 3 120 1200.5 — — — 264.9 −12.0 −117.9 — — 34.6 40.7 10 High Rotational — — —289.3 19.1 −51.4 — 41.8 Speed — — — 315.6 24.8 −51.7 — 45.8 4 25 71 1.520.0 10.0 4.1 23.0 36.0 51.9 3.1 2.0 8.4 7.6 1 Change in Time 3.4 −1.82.4 11.4 28.3 43.9 1.0 6.8 to Lift Top Ring 5 25 71 0.5 — — — 32.0 35.953.0 — — 9.1 9.5 2 Ratio of — — — 21.7 26.9 45.7 — 7.6 Rotational Speeds— — — 17.5 37.4 55.0 — 8.6 of Top Ring to — — — 25.0 57.3 81.4 — 12.7Polishing Table: 2.8 6 25 50 0.5 — — — 36.9 69.2 93.9 — — 15.0 10.8 3Ratio of — — — 6.4 26.3 44.4 — 6.6 Rotational Speeds of Top Ring toPolishing Table: 2.0 7 25 40 0.5 — — — 22.2 45.9 66.1 — — 10.4 11.7 4Ratio of — — — 27.2 60.4 82.2 — 13.0 Rotational Speeds of Top Ring toPolishing Table: 1.6 8 50 71 0.5 — — — 284.8 14.0 −91.1 — — 38.5 35.6 7Ratio of — — — 250.1 −12.4 −110.9 — 32.8 Rotational Speeds of Top Ringto Polishing Table: 1.4

In Table 1, levels of stresses in a semiconductor wafer are representedby 11 steps. The minimum level of stress is represented by 1, and themaximum level of stress is represented by 11.

It can be seen from Table 1 that Example 4 had the lowest level ofstress caused to the semiconductor wafer and thus required the smallestforces to lift the top ring. In this case, the semiconductor wafer caneasily be separated from the polishing pad. Example 5 had the secondlowest level of stress caused to the semiconductor wafer and thusrequired the second smallest forces to lift the top ring. Thus, when aratio of rotational speeds of the top ring to the polishing table islarge, the semiconductor wafer can easily be separated from thepolishing pad.

From the above experiments, it is desirable that the rotational speed ofthe polishing table is reduced to 50 rpm (50 min⁻¹) or less when thesemiconductor wafer is separated from the polishing surface. If thepolishing table is rotated at a rotational speed higher than 50 rpm (50min⁻¹), pure water or polishing liquid on the polishing table scattersunder centrifugal forces, so that effects of a hydroplaning phenomenonare weakened.

As described above, according to the present invention, even if a topring is lifted directly at a position where a polishing process iscompleted, a workpiece can readily be removed or separated from apolishing surface without an overhangining action. Thus, the workpieceis not left on the polishing surface and prevented from being cracked orscratched, which would be caused by an overhanging action.

Further, since it is not necessary to perform an overhanging action,polishing tact time can be shortened, and throughput can be improved.

When the rotational speed of the polishing table is reduced, a cleaningprocess of the polishing surface may be performed. In this case,separation of the workpiece from the polishing surface is facilitated,and conditions for the next workpiece can timely be adjusted. As aresult, throughput is improved.

In a case of a scroll-type polishing table, when the workpiece isseparated from the polishing surface, the frequency of movement of thescroll-type polishing table is reduced. In a case of a web-type(belt-type) polishing table, the speed of movement of a belt is reduced.In either case, it is possible to easily separate a workpiece from thepolishing surface.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

INDUSTRIAL APPLICABILITY

The present invention is suitable for use in a polishing apparatus forpolishing a workpiece such as a semiconductor wafer to a flat mirrorfinish.

1. A polishing method comprising: rotating a workpiece; rotating apolishing table having a polishing surface thereon; pressing theworkpiece against the polishing surface on the polishing table rotatedat a first rotational speed to polish the workpiece; separating theworkpiece from the polishing surface after said pressing; and beforesaid separating, reducing a rotational speed of the polishing table to asecond rotational speed lower than the first rotational speed to providea difference between a rotational speed of the workpiece and therotational speed of the polishing table.
 2. The polishing method asrecited in claim 1, wherein said reducing is performed during saidpressing.
 3. The polishing method as recited in claim 1, furthercomprising: supplying a polishing liquid to the polishing surface at afirst flow rate during said pressing; and before or during saidseparating, increasing a flow rate of the polishing liquid to a secondflow rate higher than the first flow rate.
 4. The polishing method asrecited in claim 1, further comprising ejecting a mixture of gas andpure water or chemical liquid to the polishing surface to clean thepolishing surface before said separating.
 5. The polishing method asrecited in claim 1, wherein said separating comprises lifting theworkpiece from the polishing surface.
 6. The polishing method as recitedin claim 5, wherein said lifting comprises reducing a lifting speed ofthe workpiece until the workpiece has substantially been separated fromthe polishing surface.
 7. A polishing method comprising: moving aworkpiece and a polishing surface relative to each other; pressing theworkpiece against the polishing surface moved at a first frequency topolish the workpiece; separating the workpiece from the polishingsurface after said pressing; and before said separating, reducing afrequency of movement of the polishing surface to a second frequencylower than the first frequency.
 8. The polishing method as recited inclaim 7, wherein said moving comprising moving the workpiece and thepolishing surface with an orbital movement.
 9. A polishing methodcomprising: moving a polishing surface relative to a workpiece; pressingthe workpiece against the polishing surface moved at a first speed topolish the workpiece; separating the workpiece from the polishingsurface after said pressing; and before said separating, reducing aspeed of movement of the polishing surface to a second speed smallerthan the first speed.
 10. The polishing method as recited in claim 9,wherein said moving comprising linearly moving the polishing surface.11. A polishing method comprising: rotating a workpiece; rotating apolishing table having a polishing surface thereon; pressing theworkpiece against the polishing surface on the polishing table;separating the workpiece from the polishing surface after said pressing;and before said separating, reducing a ratio of a rotational speed ofthe top ring to a rotational speed of the poslishing table.
 12. Thepolishing method as recited in claim 11, wherein said reducing isperformed during said pressing.
 13. The polishing method as recited inclaim 11, further comprising: supplying a polishing liquid to thepolishing surface at a first flow rate during said pressing; and beforeor during said separating, increasing a flow rate of the polishingliquid to a second flow rate higher than the first flow rate.
 14. Thepolishing method as recited in claim 11, further comprising ejecting amixture of gas and pure water or chemical liquid to the polishingsurface to clean the polishing surface before said separating.
 15. Thepolishing method as recited in claim 11, wherein said separatingcomprises lifting the workpiece from the polishing surface.
 16. Thepolishing method as recited in claim 15, wherein said lifting comprisesreducing a lifting speed of the workpiece until the workpiece hassubstantially been separated from the polishing surface.